1. Field of Invention
This present invention relates to the general area of the focussing of waves within a medium. More precisely, the invention concerns imaging procedures that require the focussing of waves through highly heterogeneous media. The applications of this type of imaging are many and varied. In particular they include under-water acoustics, telecommunications, geophysics, non-destructive testing of materials, medicine, and so on. In this last field, the focussed waves are used in imaging and in therapy for example, in particular in the case of focussed ultrasound waves.
2. Description of Related Art
The aberrations introduced by the heterogeneous media are a problem in each of these applications.
In fact, whenever a high intensity is conveyed by the waves, or whenever it is very important that only a determined zone be subjected to the waves, the aberrations are very disadvantageous, and can even prevent the use of focussed waves in certain applications due to the impossibility of achieving accurate focussing. Certain applications, in the field of the therapy for example, in fact require that the focussing be very accurate, in order not to broaden the zone of action of the focussed waves.
At present, the precision required in such applications is rendered accessible at the cost of high execution complexity, and, in the case of therapy, at the cost of invasive surgery.
In fact, in order to be able to achieve accurate focussing of high-intensity ultrasound waves in the context of brain therapy, a first solution, described in document FR 2 843 874, is to make use of the results of imaging the brain of the patient, obtained by tomographic scanning (Computed Tomography or the CT scan) performed prior to treatment. The information, in three dimensions, on the structure of the cranial bones is then used to simulate the aberrations, and to correct the signals emitted during the treatment. These digital simulations generally take quite a long time, which is incompatible with real-time working.
The patient then has to be repositioned in a magnetic resonance imaging appliance in order to allow monitoring of the real-time treatment, generally by imaging of the temperature rise.
It is then necessary for the patient to be repositioned in relation to a reference base which is identical to that used during the CT scan, in order that the structures are positioned identically. This involves a reconfiguration procedure that is generally complex. Stereotaxy frames, screwed onto the head of the patient can be used in particular.
A second solution is based on the temporal return of the waves. It consists of implanting a miniature ultrasound probe emitting ultrasound waves onto a surgical instrument that is being used during a biopsy. During the removal of tissue, the probe emits ultrasound waves in the vicinity of the tumour, and these are picked up and recorded as they exit from the cranium by a network of ultrasound transducers.
If the biopsy reveals a need for treatment, the recorded signals are then returned temporally. During their re-emission, the returned signals then focus automatically on the zone in which the biopsy was executed, that is on the tumoral zone. It is nevertheless necessary for the patient to be repositioned correctly in relation to the network of transducers and this, again, can be a difficult operation.
Then, in practice, to perform the treatment, the beam is moved electronically around the initial focal point to treat all of the tumour, point by point.
This last solution is invasive, and therefore has all of the drawbacks inherent in the invasive methods.
The use of focussed waves in the medical context, in particular the use of focussed high-intensity ultrasound waves, is therefore not favoured at present whenever elements introducing aberrations are located in the path of the waves. However ultrasound waves have some major advantages, among which are the ability to act at any time in the event of relapse or recurrence, and the ability to act on children for whom the therapeutic options are more limited than for the adult.
It should also be emphasised that the use of high-intensity ultrasound waves is an operation that is free of irradiation, and is effected by the local raising of temperature only.
All of the means that allow extension of the field of use of ultrasound waves, and more generally of the radiation-free waves, are therefore worthy of the highest interest. In particular, those means that allow the influence of aberrations to be eliminated figure amongst the means mentioned.